The concept (invented originally as more of a self-improving guideline than how it's portrayed now as a supernatural force) that positive actions see good results, and bad results come from negative actions. It is a principle of Buddhism and was created (as all principles were) for purpose of influentially commercializing the author's opinion.
Its creation's need was seen, by the philosopher, to reduce vengeance taken out upon truly deserving people, in the fear that the punisher would otherwise be remorseless. This way, the would-be grudging individual would refrain from his/her actions, with the idea that the negative person would get "what they deserve" in the long run anyhow.
Frank Castle: "I think I'll pass on my desired revenge upon my family's murderers, Karma will affect them eventually."